It is commonplace in the packaging of a wide variety of materials ranging from pharmaceutical products to instant coffee that a closure is provided in the form of a seal connected to the neck of a container and a screw cap covering and protecting the seal which provides a re-closable cap after the seal has been removed to gain access to the container. Often the closure is such that the underside of the seal has a heat sensitive adhesive coating or a meltable plastics layer covered by a metal foil. The metal foil can provide the substrate of the seal or may include a separate substrate formed from plastics material or paper. The seal is then placed against the neck of a container and sandwiched against it by the applied screw cap. An induction heating step then heats the metal foil and in turn activates the heat sensitive adhesive layer or melts the plastic layer so that on cooling, the seal bonds to the neck of the container. A difficulty often encountered by eventual users is removal of such seals from the container. Attempts have thus been made to include a tab extending sideways from the neck of the container so that the consumer can grip this to facilitate removal of the seal.
One way of overcoming this, which is proving popular at present, is the so-called “Top Tab” (Registered trademark) system, which is described fully in U.S. Pat. No. 4,961,986. This system includes a multilayer substrate which is partly de-laminated to provide a lifting tab lying wholly within the circumference of the container neck. In U.S. Pat. No. 4,961,986 this is achieved by forming the substrate from multiple layers which are adhered together over only a part of their extent. U.S. Pat. No. 5,702,015 also discloses such a seal but, in this case, the seal substrate is formed by an extrusion process in which a first layer of plastics material is extruded, followed by extrusion lamination of a second layer of release material using a third layer of extrusion material which is of the same composition to that of the first layer which integrates with the first layer where the second layer is not present. In this way the tab, which is formed by the third layer, is formed integrally with the first layer without the need for adhesive between the layers.
As shown in U.S. Pat. No. 4,961,986 the screw-cap may include some form of liner in addition to the seal material. A difficulty with a two-component system is that the seal material and the liner which are provided separately, have to be fitted inside a screw-cap in two separate operations. This naturally adds to the expense and difficulty of using the system.
In order to minimize the processing steps included in producing a seal and liner system, there has been focus on the development of a one component seal and liner system which avoids the need for two separate fitting operations.
In this regard, EP-A-1472153 describes a one component seal and liner system, for attaching into a screw cap, which includes a tab. In the product detailed, the seal portion of the system is adhered to the liner portion by means of a release layer such that the seal and liner release from each other with a peel strength in the range from 20 to 90 g at a rate of 1500 mm/min on a sample strip 25 mm wide. The adhesive used is low density polyethylene. One disadvantage of such a system is that, when fixed in a screw cap, in order that release occurs as required, it is often the case that the system needs to be rotatable within the cap rather than fixed in place. This means that screw caps which have a circumferentially extending rib are required thus increasing the costs of the overall process.
A further example of a one component seal and liner system is DE9108866 in which the seal and liner portions are adhered by means of wax for the purposes of handling and fitting the system. On heating of the metal foil in the seal portion the wax melts and is absorbed into an absorbent secondary liner whereby the seal portion and liner substantially separate from each other. On opening the seal portion remains adhered to the container and the liner remains in the cap. This system includes a tab which is formed by adhering the top layer of seal portion to the remainder of the seat across part only of the area of the seal.
A problem with this system is that the seal portion has a tendency to tear in use when a user attempts to remove the seal from a container to which it is attached by pulling on the tab.
A further problem which can be identified with such systems is that in attaching the system including the tab to a container to be sealed, an uneven level of bonding is achieved with there being a propensity for higher bonds to be formed under the tabbed portion of the liner as compared to the non-tabbed portion. There is a further danger that on heating the metal foil, the top layer of the seal will burn where the heat transferred to this layer is too great.
In WO-A-9605055 multilayer composite films having a barrier layer of amorphous carbon between a heat sealable layer and a polymeric base layer are described. The laminate may be used as part of an induction innerseal for a screw-capped container, for instance a system including an absorbent liner adhered to the top of the composite film by means of a wax layer. Upon induction heating the wax melts and is absorbed into the liner to release the adhesion of the liner. Another use of the laminates is to form top-tabbed innerseals, i.e. vessel closing assemblies including a free tab lying wholly within the circumference of the seal.
It is clear that there is a need for a vessel closing assembly which is economical to use but avoids the problems associated with the prior art.